A visit to Myanmar next month by United States Secretary of State John Kerry is set to pose a diplomatic challenge, as scepticism rises in Congress over Washington's embrace of Naypyitaw's democratic transition.

The trip will precede President Barack Obama's own visit to Myanmar in November.

Rampages by Buddhist right-wingers against minority Muslims - and the recent interrogation and detention of journalists - have soured hopes for Myanmar's transition and reforms.

Calls are emerging from Congress for Kerry to roll back part of the red carpet extended to Myanmar's quasi-civilian government - for example, Washington has lifted various sanctions and initiated limited military-to-military cooperation.

The concerns are real, but analysts point out that the dissent in Congress reflects a Republican political agenda aimed at tarnishing what Obama considers a foreign policy success.

Last Wednesday, Republican Ed Royce, who chairs the House Foreign Affairs Committee, called for measures against President Thein Sein's government, including visa bans on those involved in violence against the Muslim Rohingya minority, an end to the budding military cooperation and the reimposition of sanctions.

"It is time that we take off the rose-coloured glasses and see the situation in Burma for what it is," Royce said. "We cannot continue to lavish more incentives on the government of Burma in hopes that it will do the right thing."

His comments came soon after Mandalay was rocked by four days of anti-Muslim riots that left many injured and two dead - a Muslim and a Buddhist - before security forces regained control.

In the past two years, organised anti-Muslim violence stoked by the Buddhist right-wing 969 Movement has left many dead, mostly Muslims.

It has driven nearly 140,000 Rohingyas in Rakhine state from their homes and communities into squalid refugee camps.

President Thein Sein has repeatedly condemned the violence, and vowed to prosecute those responsible. But in practice, there seems to be no attempt to curb more vocal symbols of the resurgent Buddhist nationalist movement - such as Mandalay-based monk U Wirathu.

Critics say the government is in effect complicit, in part because it shares wide cultural prejudice against Muslims. Even opposition leader Aung San Suu Kyi has been ambiguous on the issue.

Analysts say the government fears that moving against Buddhist nationalists could prompt monks to turn against it. Likewise, Suu Kyi cannot risk antagonising her Buddhist voter base by standing up for the rights of Muslims.

The Obama administration has acknowledged the nascent nature of the transition and continuing concerns over human rights abuses. But diplomats and analysts say that taking punitive diplomatic actions against Myanmar over these issues would be of little or no help.

A Myanmar-based European diplomat told The Straits Times: "If you want to effect change, you must bring the most isolated and conservative people closer. The military-to-military engagement is the single most promising approach."

Noting the "flurry of negative comments from the US", Derek Tonkin, former British ambassador to Thailand and noted Burma expert, wrote on his Network Myanmar website that there was cause for concern over issues related to the Rohingya and planned legislation restricting interfaith relations.

But the concern being expressed in Congress was really "a Republican campaign to deny the administration any political kudos for what the latter claims to have been a foreign policy success".

Kerry will have to find common ground in Myanmar, while addressing concerns in Washington, even if many are politically driven.

Analysts have urged the US to keep an eye on the big picture.

In an e-mail, Yangon-based Richard Horsey, an independent consultant and a former top United Nations official in Myanmar, wrote that developments there were "hardly surprising in a country emerging from five decades of authoritarianism, civil war and economic malaise".

Even so, he maintained: "There is no question that Myanmar remains a fundamentally more open and liberal place than it was three years ago."

For a country like Myanmar, the path of reform would be "bumpy and winding", he said. "Expectations must remain high regarding Myanmar, and scrutiny maintained, but with a sense of realism about what is realistically achievable."